Many electronic devices, such as computers and many household appliances, commonly require that an alternating current line voltage be converted to one or more direct currents. Such electronic devices are ordinarily powered by a category of power supplies referred to as "switched power supplies," "switch mode power supplies" or "switching power supplies." All such power supplies will be referred to herein as "switching power supplies."
Many switching power supplies include a rectifier bridge which converts the input AC voltage to an unregulated DC voltage, also known as a rectified line voltage. These supplies also usually include a filtering capacitor coupled to the output of the rectifier bridge. This bridge-capacitor configuration leads to high currents near the peak of the AC voltage cycle, and substantially zero current around the zero-crossing points of the voltage cycle, causing a highly non-sinusoidal current waveform. As is known in the art, this non-sinusoidal current waveform can be decomposed into a set of harmonic sinusoids, each having an oscillation frequency equal to an integer multiple of the fundamental frequency of the input voltage. For example, with a 60 Hz input voltage frequency, such a current waveform would have harmonic frequencies at 60 Hz (the fundamental), 120 Hz, 180 Hz, 240 Hz, 300 Hz, etc.
As is known in the electrical power art, current harmonics above the fundamental frequency of the voltage do not contribute to the power drawn from a typical AC voltage source, with the result that the actual power drawn by the power supply is lower than the apparent power drawn. The explanation for this phenomenon is straightforward. The apparent power drawn is defined to be the product of the RMS voltage times the RMS current. The actual or "true" power is the integrated product of the instantaneous voltage and current, V*I, over a voltage cycle divided by the cycle's period. In accordance with the Fourier Series Theory, the integral of a fundamental harmonic with any other harmonic over one fundamental period is zero. Thus, assuming the input voltage is a sinusoid, the true power is simply the integral of the voltage sinusoid with the fundamental current harmonic, and the higher order current harmonics do not contribute to the true power. By contrast, all of the current harmonics contribute to the RMS value for the current; thus, the higher order harmonics do contribute to the apparent power drawn from the AC voltage source. Similarly, the apparent power is also higher than the true power when there is a phase difference between the voltage sinusoid and the fundamental current harmonic.
The distinction between apparent power and true power is important because power sources are rated according to the apparent power drawn rather than the true power drawn. As a basis of comparison, the true power and apparent power drawn by a device are divided to form a ratio called the "power factor," which is often abbreviated as "pf." The power factor of a device is broadly defined as: ##EQU1## The rectifier bridge and capacitor combination described above has a typical power factor in the range of 0.6-0.7.
Power factors less than about eighty percent can pose barriers to the performance or improvement of many types of electronic devices that operate on direct current, including such devices as personal computers, minicomputers, and appliances using microprocessors. For example, the high current peaks associated with low power factors can cause circuit breakers on the line voltage to trip, which limits system design in terms of the functional load it places on a standard line. Additionally, the harmonics associated with the high, non-sinusoidal current peaks result in power-line distortion, noise, and electromagnetic interference (EMI). In general, improving the power factor of the device reduces the harmonic content and electromagnetic noise. To address these problems, many regulating organizations, such as the International Electrotechnical Commission ("IEC"), have instituted or are planning to institute standards for controlling the harmonic content and electromagnetic noise generated by many electrical devices.
In order to raise power factors and attempt to comply with such regulations, manufacturers of computer systems and power supplies have begun to develop circuits for raising power factors and eliminating harmonic distortion. Such circuits are often referred to as power factor correction circuits. A number of power factor correction circuits exist in the prior art.
Existing power factor correction circuits are often relatively expensive because they use relatively complex circuit designs having active switching elements. While complex, active circuits can lead to substantial improvements in power factors, these circuits can be prohibitively expensive for a number of applications, particularly low power (&lt;300 W) applications. Currently, the harmonic content and power factor of low power applications are not regulated by the IEC. The IEC is, however, considering regulating the harmonic content of such low power applications. If implemented, the current IEC proposal will effectively require power factors of at least about 0.80, well above the 0.6-0.7 value of the simple bridge-capacitor combination discussed above.
Accordingly, there is a need for a relatively inexpensive power factor correction circuit that will raise the power factor of a power supply and reduce the power-line distortion, noise, and EMI associated with a low power factor.